This disclosure relates to drilling wellbores in formations below the bottom of a body of water. The disclosure relates more particularly to wellbore pressure control apparatus used to prevent uncontrolled escape of fluids from such wellbores and the accompanying hazards associated with such uncontrolled escape.
Wellbores drilled through formations below the bottom of a body of water may use a conduit called a riser that extends from a drilling platform on the water surface to a wellhead or pressure control devices (blowout preventers—BOPs) proximate the bottom of the body of water. The riser may provide a guide for a drill string used to drill the wellbore and may serve as a conduit to return to the drilling platform some of all of a volume of drilling fluid (“mud”) used in the drilling process. The mud is pumped from the drilling platform through the drill string. The mud column in the riser provides hydraulic pressure (related to the density of the mud, the vertical length of the riser, and hydrodynamic properties of the mud) to prevent entry into the wellbore of fluid from formations exposed by drilling the wellbore. The mud column constitutes a primary well barrier and in most cases overbalances formation pore fluid pressure. In some cases, the hydraulic pressure is insufficient to prevent flow of some fluids into the wellbore. Inflow of gas into the wellbore is particularly hazardous because as gas travels upwardly in the wellbore, and ultimately in the riser, it expands as the hydraulic pressure decreases with respect to vertical depth. Such expansion can then produce a self-progressing, increasing displacement of mud from the wellbore, further reducing hydraulic pressure in the wellbore and enabling more fluids to enter the wellbore. In such event, the primary well barrier is then lost and a well pressure control event may occur.
U.S. Pat. No. 8,413,724 issued to Carbaugh et al. describes a device for diverting gas in a riser. The device includes a user-controlled sealing element disposed in the riser that closes the annular space between the drill string and the riser. When the annular sealing element is closed, gas may be diverted into one or more conduits to enable venting the gas in a controlled manner. The device disclosed in the '724 patent requires the user to operate the annular sealing element. It is possible for gas to enter the wellbore undetected such that user operation of the annular sealing element is delayed enough to create a hazardous condition in the wellbore and/or the riser.
U.S. Pat. No. 9,068,420 issued to Rajabi et al. describes a passive choke that may be inserted into a riser. The passive choke provides a relatively small cross-section annular space between the riser and the drill string such that upward flow of fluid in the riser is limited. Drilling mud is returned mainly through a separate mud return line in fluid communication with the interior of the riser below the passive choke. No user action is required to make use of the passive choke disclosed in the '420 patent. However, full fluid closure of the wellbore still requires user operation of the BOPs or further pressure control devices located in the riser or proximate the drilling platform.